This invention relates to data communication over Digital Telephone Networks (DTN). In particular, the invention relates to a signaling system used in a data distribution system that consists of a data source, or server, directly connected to the DTN, without any Analog-to-Digital (A/D) conversion or Digital-to-Analog (D/A) conversion, and at the receiving end, a client, or subscriber, connected to a DTN in the normal fashion.
Presently, typical modems used to communicate over the public telephone system (one present standard for such communications is detailed in the International Telecommunication Union, Telecommunication Standardization Sector ("ITU-T") Recommendation V.34 (1994)), represent binary data by an analog waveform that is modulated in response to the binary data. The waveform is in turn analyzed at a receiving modem to recover the binary data. For modem signals transmitted over the public telephone system, such analog waveforms are treated by central office switches as if the waveforms were analog voice signals.
FIG. 1 shows a typical communication system presently used for data communication. The basic elements of the subscriber loop connection are a modem 62 that is connected by an analog line 64 to a local switch 66, which terminates the switched digital telephone network 60. The modem 62 is typically located at the subscriber's premises and includes a receiver 68 and transmitter 70. As shown in FIG. 1 the receiver 68 and the transmitter 70 are coupled to the analog line 64 by a hybrid 72. The transmitter 70 converts input digital data 74 into analog signals that are passed through the hybrid 72 and transmitted over the analog line 24 to the local switch 66. Likewise, the receiver 68 converts input analog signals, which pass from the analog line 24 through the hybrid 32, into digital data 36.
At the local switch 66 end of the subscriber loop, analog signals from the line 64 are directed through a hybrid 78 to an analog-to-digital converter 80. The analog-to-digital converter 80 samples the analog signals converting them into a digital data stream for transmission through the switched digital telephone network 60. For transmission in the opposite direction, a digital data stream is applied from the digital telephone network 60 to a digital-to-analog converter 82. The digital-to-analog converter 82 converts the data stream into analog signals that are passed through to the hybrid 78 to the analog line 64, for transmission to the appropriate subscriber.
Typically, the waveforms are digitized into eight bit octets by a codec A/D converter at the central office, and the octets are transmitted in digital format between central offices until they are converted back to an analog signal by a D/A codec at the central office that is connected to the receiving subscriber loop. The data rate attainable by a modem operating in such an environment is limited by numerous factors including, in particular, the codec sample rate and the number and spacing of quantization levels of the codec convertors at the central office switches.
The effect on an analog signal associated with sampling the signal amplitude and representing the sample by one of a finite number of discrete (digital) values is generally referred to as quantization noise. Most telephone switches utilize voice codecs that perform nonlinear A/D and D/A conversions known as .mu.-law or A-law conversion. In these conversion formats, the 8-bit codec codewords, also referred to as octets, represent analog voltages that are nonlinearly spaced. This type of conversion performs well for voice signals intended for a human listener (especially when transmitted over a noisy line), but have a negative impact on modulated analog waveforms associated with modems. Specifically, codecs that adhere to these standard nonlinear conversion formats implement nonlinearly spaced quantization levels, and have the effect of increasing quantization noise which is detrimental to modem signals.
The method and apparatus described herein is for use in a system for conveying digital data across a digital telephone switch to an analog subscriber, where a data source at one end of the link is connected directly to the digital telephone network (DTN) without undergoing an analog-to-digital conversion. At the other end of the link, the analog subscriber is connected to the telephone network in the standard fashion. This type of system may operate at data rates much higher than systems whose signals undergo an analog-to-digital conversion of a transmitted analog signal and a subsequent digital-to-analog conversion at the receiving end, due at least in part to an associated decrease in quantization noise. In such a system, data is transmitted through the telephone switching network in a digital format (via, e.g., T1 lines) and is only converted to an analog voltage when it reaches the central office connected to the subscriber's local loop.
FIG. 2 shows a block diagram of such a data distribution system. The system includes a data source 10, or server, having a direct digital connection 20 to a digital telephone network (DTN) 30. A client 40 is connected to the DTN 30 by a subscriber loop 50 that is typically a two-wire analog line. The DTN routes digital signals from the data source 10 to the client's local subscriber loop without any intermediary analog facilities such that the only analog portion of the link from the server to the client is the client's local loop. The analog portion thus includes the channel characteristics of the local loop transmission line plus the associated analog electronics at both ends of the line. The only D/A converter in the transmission path from the server to the client is the one at the DTN end of the client's subscriber loop. For the reverse channel, the only A/D converter in the path from the client to the server is also at the telephone company's end of the client's subscriber loop.
The communications format used by the invention described herein is known as pulse amplitude modulation. Essentially, the codecs are used to generate the varying amplitude pulses that are sent over the subscriber loop. Each octet sent to the subscriber's local loop is converted to an analog voltage by the DTN's codec. Thus each octet may be used to generate a desired voltage amplitude during that time interval, until the next octet is received (presently, octets are transferred at a rate of 8 K Hz).
An impediment to using the full capacity of a data link that has direct digital access to the telephone system and that uses codec codewords to produce pulse amplitude modulation is that the DTN may periodically introduce errors in the binary data due to the use of what is known in the telecommunications industry as "robbed bit signaling" (RBS). Additionally, the noise level may prevent a receiver from being able to distinguish between all of the possible codec output levels.